1. Field of the Invention
The present invention relates to a method of preserving a fuel cell membrane electrode assembly in which an air electrode and a fuel electrode are respectively stacked onto the surfaces of a polymer electrolyte.
2. Description of the Related Art
Recently, a fuel cell has attracted a great deal of attention as a clean power-generating resource. A variety of types of fuel cells are provided, and a polymer-electrolyte-type fuel cell is among them.
The polymer-electrolyte-type fuel cell has a so-called fuel cell stack (tandem cell) in which a plurality of smallest units, each being called a “unit cell” and generating power, are stacked in series. By providing the fuel cell stack with a unit for providing oxygen and fuel or a unit for cooling down the tandem cell, a desired power (voltage) can be obtained through a reaction between hydrogen and oxygen in each unit cell.
The unit cell has a separator for conducting electricity and separating the assemblies which are adjacent to each other when unit cells are stacked. It is the fuel cell membrane electrode assembly that mainly controls the reaction between hydrogen and oxygen.
The fuel cell membrane electrode assembly includes an electrolyte made of a polymer ion-exchange membrane that is similar to a fluoropolymer ion-exchange membrane that has a sulfonic group. The fuel cell membrane electrode assembly also includes a cathode catalyst layer that becomes an air electrode and an anode catalyst layer that becomes a fuel electrode when placed on each surface of the electrolyte. For example, metal made of platinum and ruthenium is used for the anode catalyst layer while platinum is used for the cathode catalyst layer.
The fuel cell membrane electrode assembly with the structure as described above causes a reaction between oxygen and hydrogen as follows: hydrogen gas provided for the fuel electrode is changed into hydrogen ion in the anode catalyst layer; and the hydrogen ion is moved, in a state of hydration, to the oxygen electrode's side through the electrolyte. The ion then reacts with oxygen and electron and generates water in the cathode catalyst layer. By repeating the reaction, the fuel cell membrane electrode assembly generates power (voltage).
Such a polymer electrolyte fuel cell is manufactured almost in sequence from a manufacturing of a unit cell, an assembling of a fuel cell stack through to a final process of assembling a fuel cell. Therefore, it is possible to manufacture a fuel cell that has satisfactory functions.
Today, an era of mass production of fuel cells is about to coincide with the spread of fuel cell use. There is a good possibility for a necessity to preserve, for a long time, without degrading the function thereof, the parts used for assembling a fuel cell in order to maintain the desired functions throughout the process of manufacturing. For example, the following method is conceivable for preserving a fuel cell stack in an atmosphere that is purged of air (oxygen): purging with the use of inert gas, or purging with the use of moisture, so that the air (oxygen) remaining in a fluid channel which is placed in a separator that transmits oxygen gas and hydrogen gas can be eliminated (see reference to Japanese Laid-Open Applications No. 2002-93448 and No. 06-251788). Another preservation method involving using an oxygen-absorbing substance is suggested as a technique of preserving a fuel cell stack in an atmosphere that is purged of oxygen (see reference to Japanese Laid-Open Application No. 2000-289380).
It is possible to preserve the fuel cell stack for a long time, using the conventional method. Along with the progress in general use of fuel cells, however, in some cases, only fuel cell membrane electrode assemblies are manufactured and transported to a distant place. In this case, the conventional method is not effective.
After diligent research through the years in view of the conventional techniques, the inventors of the present invention have comes, to discover a cause of the problem generated in the preservation of fuel cell membrane electrode assembly.
According to the research, the cause of the problem turns out to be the use of alcohol in the process of manufacturing a catalyst that makes up a membrane electrode assembly. For example, acetylene black carbon powder that supports platinum-ruthenium metal particles or platinum particles is used as a catalyst powder, and a pasty catalyst is manufactured by dispersing this catalyst powder onto ethyl alcohol that contains perfluoro-carbon sulfone acid powder. The pasty catalyst is spread over a non-woven fabric made of carbon. A catalyst layer is formed in this way. A membrane electrode assembly is manufactured by sandwiching the electrolyte with two catalyst layers whose surface on which the catalyst is applied faces toward the electrolyte.
The alcohol remains, however, on the non-woven fabric even after the membrane electrode assembly is manufactured. If the membrane electrode assembly is preserved in such condition, oxide is generated as a result of the reaction between oxygen in the air and the alcohol, which affects the catalyst. The obtained observation is that the catalyst layer itself may be degenerated due to the long-term preservation of the membrane electrode assembly.
Another observation is that, in some cases, a dust such as an organic compound contained in the air may stick to the membrane electrode assembly depending on the environmental condition in a factory or a stock room, and the catalyst may be degenerated if an unnecessary organic compound adheres to the membrane electrode assembly for a long time.
In the case where metallic (transition metal in particular) particulates reach the electrolyte, the metal particles are ionized since the electrolyte is strongly acid. When the electrolyte to which ionized metallic particles adhere is provided to the fuel cell so that the fuel cell is activated, hydroxyl radical is generated as a result of the reaction between hydrogen peroxide generated due to the gas that cross leaks from the electrolyte or the secondary reactions, and the ionized metal adhering to the metal particulates. The electrolyte is decomposed by the generated hydroxyl radical. The observation shows that, after the decomposition of the electrolyte, the electrolyte increasingly cross leaks so as to accelerate the decomposition of the electrolyte resulting in decreases in film pressure of the electrolyte that are evident to the extent that power cannot be constantly generated.
It has also been observed that after the exposure to the oxygen in the air, each of the catalyst layers rises to a high voltage that is close to 1V. This accelerates oxidization of a metallic catalyst such as carrier carbon, platinum and ruthenium in the catalyst layer. Due to the oxidization, the catalyst layer loses its function as a catalyst or the catalyst melts out of the catalyst layer which makes the layer deficient.
Moreover, it turns out that the change in humidity in the environment where the fuel cell membrane electrode assembly is preserved causes damage to the electrolyte or to the catalyst layer after the repetition of expansion and shrinking of the electrolyte.
The inventors also discovered that in the case where the fuel cell membrane electrode assembly falls into one of the above cases, the fuel cell made of such fuel cell membrane electrode assembly can be a cause of degradation in initial characteristic such as voltage and/or current characteristic or a cause of degradation in serviceability of the fuel cell over a long term.
It has also been found that the expansion and shrinking of the electrolyte also causes change in size, which renders it difficult or impossible to build up a unit cell.
Note that in the case where an oxygen-absorbing substance is placed in a package that has low oxygen permeability, the problem of oxidization can be prevented. However, the oxygen-absorbing substance must be carefully selected because in some cases a substance that accelerates decomposition of electrolyte may be emitted from the oxygen-absorbing substance.